It's so that you're fairly giving reps to all the members of the community, rather than just a few, most likely to avoid is being easier for friends to just spam each other's posts with reps so they can get more gems.

It's so annoying, I can't imagine there's many people sad enough to get friends to rep them loads so they'll get gems in a pointless arbitrary reward system.
But there are members who often say things I like and I want to rep them a lot and I'm not allowed